Various filter systems have been developed to filter water and wastewater. Typical filter systems include but are not limited to upflow filters, upflow clarifiers, a downflow filter, downflow clarifiers, bi-flow filters as well as various combinations of the aforementioned filter systems. The terms upflow filter and upflow clarifier refer to filter systems in which influent to be filtered is directed in an upward path to remove impurities. Conversely, the terms downflow filter and downflow clarifier refers to filter systems in which the influent is directed in a downward path to remove impurities. Bi-flow filters refer to filter systems in which the influent is directed in both a downward path and an upward path to remove impurities. The upflow filters/clarifiers, downflow filters/clarifiers and bi-flow filters include one or more filter beds formed from one or more layers of filter media. The filter media may be supported by one or more layers of gravel. Alternatively, the one or more gravel layers may be replaced with a porous plate or other direct retention systems.
In a combined system, for example, an upflow filter/clarifier followed by a downflow filter, the influent is initially directed upwardly through the upflow filter/clarifier to remove a predetermined percentage of the impurities in the influent and then directed downwardly through the downflow filter to remove the remaining impurities to within an acceptable limit. Where the filter system includes multiple downflow filters connected in series, the influent is directed downwardly through each of the downflow filters one after the other to remove the impurities within an acceptable limit.
Regardless of the type of granular bed filter system and/or the type of the liquid to be filtered, it is common for the filter system to include an underdrain. The underdrain directs and receives fluids during operation of various cycles of the filter system including the filtration cycle (also referred to as a service run) and a washing cycle. The washing cycle may be performed by directing the washing fluid in a path opposite to the path of influent during the filtration cycle or the washing cycle may be performed by directing the washing fluid in the same path as the path of the influent during the filtration cycle. The washing fluids may include influent, pre-filtered water, air or other suitable gas. A key function of the underdrain is to evenly distribute the washing fluid through the filter bed during the washing cycle so that at least a significant amount of the impurities trapped in the filter can be removed. In the event that the underdrain does not evenly distribute the washing fluid through the filter bed, the filter bed will likely not be properly cleaned. At a minimum, this will cause much shorter filtration cycles and more washing cycles. This is undesirable, as the filter system typically cannot operate in the filtration cycle during a washing cycle.
There are many different types of underdrain systems currently marketed for use in water filtration. Some underdrains are used for water only backwash and some are capable of introducing air only, water only, or simultaneous liquid and air.
A common type of underdrain is the “lateral” style. This type of underdrain is typically made of injection molded or extruded plastic or extruded clay. The underdrain blocks are arranged in rows termed “laterals”. The laterals are typically spaced on approximately 12 inch centers leaving an approximately 1 inch space between the laterals. There are two common types of combined air/water lateral type underdrains. One type is configured to introduce air and water into a center compartment that directs the air and water through separate internal orifices into two adjacent, outer compartments. The air and water escape only from the two adjacent, outer compartments into the filter bed from combined air/water orifices in the two adjacent, outer compartments. The two adjacent, outer compartments form in essence dual internal laterals that extend generally the length of the single lateral. This type of system has the limitation that the air and water in the shared compartment is always at the same pressure and a distinct air/water interface is formed within this compartment. The water orifices are located at a lower elevation than the air orifices. The air/water interface must be maintained between the air and water orifices or gross maldistribution of air and/or water will result. As the pressure in this compartment increases, the air/water interface is pushed down lower in the underdrain lateral. Thus, the pressure of the air is limited to the height of the underdrain and the distance between the air and water orifices. In practice, this configuration limits the driving force of the air to relatively low pressures. These types of underdrains are very susceptible to maldistribution problems due to uncontrolled air, flow surges, wave action of the air/water interface and require very tight installation tolerances.
The second common type of air/water lateral employs dedicated air only and water only compartments that distribute the air and water thru separate orifices without the intermixing of the air and water within the lateral. This type of system has the limitation that the air and water distribution orifices that disperse their respective fluid into the filter must be located in their respective distribution chambers. In practice, the size of the various distribution chambers force the distribution orifices into distinct sections of the underdrain and limit the minimum distance between orifices. This configuration results in poor distribution across the underdrain lateral.
Both the first and second types of underdrain laterals are incapable of directing a combination of liquid and air from one compartment and only liquid or air from another compartment.